The depth of oil wells drilled with current technology can reach tens of thousands of feet. The wells may be non-linear in order to increase exposure to the production zone. Maximum depth is limited by the mechanical strength of the drill pipe. In particular, the depth is limited by the capability of the drill pipe to withstand the compressive, tensile, torsional, bending, and pressure differential forces required to create the borehole. The pipe is subjected to torsional forces due to the torque required to overcome both friction against the formation and the torque to rotate the drill bit. The decrease of torsional stiffness due to the extended length of the drill string in deep wells and the friction against the formation can even cause stick-slip effects which, in extreme cases, can lead to self-unscrewing of drill pipe joints, drill bit damage, BHA vibration, and other undesirable results. In cases where surface or intermediate casings are already in place, friction between casing and drill string can wear through the casings at the pressure points of bends, resulting in either formation fluids entering the well or lost circulation. Extensive forces of the drill pipe against the mud cake wall can lead to differential sticking and loss of equipment.
It is known to reduce drill string friction by using stabilizer subs having a non-rotating sleeve. The bearing surface between the sleeve and the drill pipe includes a set of sliding or rolling element bearings. While such stabilizer subs reduce friction, they are relatively complex and costly. Because of the complexity, such stabilizer subs are more likely to fail than simpler devices. Ball bearing packages, for example, are particularly subject to degradation and failure in a borehole environment. Non-rotating sleeves are also problematic when they become jammed against the formation downhole because the bearings themselves inhibit the use of torsionally applied force to free the sleeve. The overall cost of use of such subs can be considerable because it is a multiple of stabilizer sub unit cost and the number of required subs. On a 30,000 ft drill string, 500 such stabilizer subs would be needed if they were used every 60 ft.
Another way of reducing drill string friction is described by J. G. Boulet, J. A. Shepherd, J. Batham: Improved Hole Cleaning and Reduced Rotary Torque by New External Profile on Drilling Equipment, IADC/SPE Drilling Conference, New Orleans, La. No. 59143, February 2000 (“Boulet”), herein incorporated by reference in its entirety. According to Boulet, an external drill string sub profile includes a hydrodynamic bearing. This bearing provides a film of pressurized fluid between the drill string and the borehole. However, the shape of the hydrodynamic sub requires very complex and expensive machining, rendering this solution uneconomical and impractical with current manufacturing techniques. Furthermore, the Boulet hydrodynamic sub design only reduces friction when the drill string is rotating, and thus it will not provide assistance for restarting rotation after a new joint of drill pipe has been added at the surface.